In the field of radio communication, there has been actively employed in recent years a MIMO (Multiple-Input and Multiple-Output) transmission scheme, which enables increased speed and improved quality of signal transmission by carrying out transmission/reception on both a transmitter side and a receiver side using a plurality of antennas.
Further, there is known in the art (for example, Patent Document 1) a massive-MIMO transmission scheme in which there is used a large number of antenna elements so as to achieve a further increase in speed and a reduction in interference in signal transmission, (for example, 100 elements or more) in a high-frequency band (for example 10 GHz or more), and use of which also enables miniaturization of antennas and a wide bandwidth to be attained.
With massive-MIMO, there can be achieved advanced beamforming (BF) by use of a large number of antenna elements, compared to conventional MIMOs. Beamforming is a technique of controlling the directivity and shape of beams (where transmission beams correspond to transmission antennas, and reception beams correspond to reception antennas) by use of a plurality of antenna elements. With MIMO, phase and amplitude can be controlled for each antenna element, thus the larger the number of antenna elements used, the greater the freedom of beam control.
Mathematically, beamforming applied to transmission signals is represented as a multiplication of a vector indicating a transmission signal by a beamforming weight matrix (hereafter, sometimes referred to as BF weight matrix). A BF weight matrix is represented as a matrix that includes a plurality of beamforming weight vectors (hereafter, sometimes referred to as BF weight vectors) as a component. Furthermore, it is noted that hereafter BF weight matrices and BF weight vectors are sometimes collectively referred to as “BF weight”.
As one mode of beamforming, fixed beamforming is exemplified. In fixed beamforming, a beamforming weight (fixed beam) for use is selected from among a plurality of beamforming weights prepared in advance. In fixed beamforming, beamforming that controls fixed beams and coding that achieves compensation for multiplexing between a plurality of streams (transmission-side precoding and reception-side postcoding) are carried out separately.